os: fix Rename on Plan 9
os: fix Rename on Plan 9

Rename should fail when the directory doesn't match.
It will fix the newly introduced test from cmd/pack
on Plan 9.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/65270044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang